This invention relates to super-conducting electro-magnets for use in magnetic resonance imaging (MRI) apparatus.
Super-conducting electro-magnets for MRI apparatus comprise windings immersed in liquid helium so that they are cooled to temperatures of about 4.degree. K at which they become super-conductors. A helium vessel containing the windings is positioned within a vacuum chamber which serves to reduce heat gain by convection and in order to reduce heat gain by radiation, thermal shields are positioned between the helium vessel and the vacuum chamber. The thermal shields, are usually cooled by a refrigerator which in known apparatus is provided in a refrigerator turret. Such an arrangement is described in European patent specification EP0260036. The refrigerator turret extends into the vacuum chamber to facilitate thermal coupling of the shields to the refrigerator. In order to provide access to the windings for current injection purposes, and access to the helium vessel for filling and topping up purposes, in known apparatus a service turret is additionally provided.
Although generally satisfactory in operation, this known arrangement has the disadvantage that it tends to be expensive to fabricate and time consuming to assemble. It also has the disadvantage that a current probe must be inserted to make electrical contact with a connector in the helium vessel when it is desired to inject current into the windings. The insertion of the current probe is an undesirable operation because the turret must be opened to atmosphere, and air/water vapour from the atmosphere could enter the turret where it would freeze on the cold surfaces, creating a blockage. In addition, inserting the current probe into a magnet which is energised can cause the magnet to quench if insufficient care is taken to cool the probe fully before engagement with the connector on the windings. Magnets with the known service turret may be fitted with permanently connected current leads, but because these are cooled only by gas boiling off from the helium vessel, they impose a considerable heat load onto the helium vessel by virtue of having to have good electrical conductivity (and therefore good thermal conductivity) to connect electrical current to the windings. The reasons for this are well understood by those skilled in the art, and further detailed explanation is therefore believed to be unnecessary.